Bagworm Control Efforts Should Start Now

Bagworm Control Efforts Should Start Now

May 22, 2009

Figure 1. Bagworms on juniper.

Figure 2. Spruce tree severely damaged by bagworms.

Bagworm Life Cycle

Bagworms overwinter as eggs within bags fastened to twigs. Eggs hatch from mid-May to early June. Immediately after hatching, some of the caterpillars release a streamer of silk and are blown by the wind, establishing new infestations on nearby trees.

Others begin to spin tiny (l/8 inch) protective cases or "bags" around themselves. These bags are constructed of silk and fragments of needles or leaves. As bagworms grow, leaf fragments are added to bags, which often grow to 2 inches in length by the end of the summer. Larvae feed until late August or early September.

Male bagworm moths emerge in September and mate with the wingless females through the bag opening. Female moths deposit their eggs within their own bags, drop to the soil and die. Each female can produce 500 to 1000 eggs. There is only one generation per year.

Timing is everything in successfully controlling bagworms before they cause irreversible damage to your windbreaks. Control measures implemented from now to mid June will be most effective.

The bagworm, Theridopteryx ephermaeformis, feeds on the foliage of a wide variety of trees and shrubs, but is of particular concern for evergreens, especially junipers. Left untreated, a severe bagworm infestation can destroy an entire windbreak of mature evergreens in just two to three years. Less severe injury will slow growth and stunt plants.

Fortunately, bagworms are relatively easy to manage if control measures are timely.

Identifying Bagworm Damage

Bagworms feed on most coniferous plants and on many deciduous trees and shrubs. Common evergreen hosts include juniper, arborvitae, spruce, and pine. The earliest sign of bagworm injury in an evergreen is brown or stressed needles at the tips of branches. This is caused by tiny, first-stage bagworm caterpillars etching needle surfaces as they feed. These plants can be severely stressed or killed. Bagworms are especially damaging to conifers because destroyed foliage is not regenerated.

Windbreaks Add Value

University of Nebraska–Lincoln research results illustrate the economic value of windbreaks and the potential long-term loss from not treating pests such as the bagworm.

Typically, windbreaks increase crop yields by 5%-7% for a distance of 8 to 15 times the height of the windbreak.

Wheat fields with windbreaks yield an average 15% more than fields without windbreaks, according to a 30-year UNL study.

A windbreak can add $60,000-$70,000 in economic value to your operation over its life, even after subtracting planting and maintenance costs.

Windbreaks protecting farmsteads can reduce energy use by 20%-30%.

Bagworm Control

Mechanical Control. Control bagworms on smaller trees and shrubs by removing bags during winter and early spring before egg hatch begins in late May. Destroy bags by crushing or immersing them in soapy water. If bags containing larvae are discarded on the ground, the larvae may return to host plants.

Natural Enemies. Bagworms have many natural insect enemies which feed on them during their larval or pupal stages. A recent study at the University of Illinois at Urbana-Champaign showed that planting asters and daisies near bagworm-infested trees provided shelter and nectar for beneficial insects and reduced bagworm numbers. If insecticidal control is needed, select reduced-risk products that have minimal impact on these natural insect enemies. Birds, especially sparrows and finches, are important predators of bagworms in late summer.

Chemical Control. Insecticides are most effective when applied during the early stages of bagworm development. Spray from late May to mid-June when bags are less than 1/2 inch. Insecticides containing Bacillus thuringiensis (Bt), spinosad, or neem oil (azadirachtin) and insecticidal soaps are quite effective against young bagworm larvae, but may require repeated applications. These products generally have minimal impact on beneficial insects.

Insecticides applied later in the summer when bagworms are larger likely won't be as effective. By late August, chemical control is no longer feasible, as most bagworms will have ceased feeding and pupated within their bags.

Insecticidal spray applications require thorough coverage to penetrate the canopy and contact the feeding bagworms. Use ground equipment that delivers a high spray volume and pressure. Aerial applications may not provide thorough enough coverage, leading to less than satisfactory bagworm control.